1. Field of the Invention
The present invention relates to an image forming apparatus that can improve throughput of a sheet without enlarging a driving source when images are formed while the sheets are continuously conveyed.
2. Description of Related Art
Conventionally, in an image forming apparatus such as a copying machine, a printer, a facsimile, and a multi function peripheral thereof, sometimes a sheet feeding apparatus that stacks many sheets to separate and feed the sheet one by one can be attached as an option.
As illustrated in FIG. 5, in the sheet feeding apparatus, many sheets are stacked on a sheet stacking portion, and a feeding roller 100 and a separation roller 101 feed and separate the sheet one by one from the topmost sheet. The fed sheet is delivered to an image forming apparatus main body by pairs of conveying rollers 102, 103, 104, and 105, and an image forming portion 106 forms an image.
At this point, in the image forming apparatus main body, after the continuously-conveyed sheet is temporarily stopped, and the sheet is conveyed toward the image forming portion at predetermined timing, thereby performing an operation (hereinafter referred to as “pre-registration operation”) for adjusting an interval between sheets. Therefore, rotation of the conveying roller 105 is temporarily stopped based on a time a sheet passing sensor 107 detects a leading end of the sheet.
Accordingly, the leading end of the sheet that is being nipped and conveyed by the conveying rollers 105 is stopped at a predetermined position on a conveying path, and the control is perform to start the rotation of the conveying roller 105 again after a predetermined time elapses.
In the sheet feeding apparatus that stacks a large number of sheets, the sheet leading end is not always located at the normal stack position. That is, the sheet that is returned to a large-capacity deck by the separation roller 101 and the sheet leading end is located near the feeding roller 100, and the sheet that is located near the feeding roller 100 are also fed. Therefore, a position of the sheet leading end is varied in starting the sheet feeding, and it is necessary to eliminate the variation in front of a registration roller 108 located immediately before the image forming portion (maximum assumed time variation TT).
Specifically, as illustrated in FIG. 6, after the sheet leading end is detected by the sheet passing sensor 107, the conveying roller 105 is temporarily stopped to fix a stop position of the sheet leading end at a time the sheet leading end reaches a predetermined position A on the conveying path. Then a time B (B=(distance C to registration roller 108)/(sheet conveying speed β of conveying roller 105 until sheet reaches registration roller 108)) at which the sheet leading end is expected to reach the registration roller 108 is computed. Therefore, the control is performed to restart the conveying roller 105.
At this point, the variation is canceled because the control is performed to restart the conveying roller 105 after a predetermined time TR (TT<TR) the maximum assumed time variation TT can sufficiently be absorbed.
Then the sheet reaches the registration roller 108 in the stopped state through a pre-registration roller 109, and the rotation of the registration roller 108 is started at a constant process speed α in synchronization with image formation timing, thereby delivering the sheet to the image forming portion to transfer toner image to the sheet.
When a plurality of sheets is continuously conveyed, the feeding of the sheets subsequent to the first sheet is started after a predetermined time elapses since the previous sheet is restarted from the conveying roller 105. This is because the collision of the leading end of the next sheet with a rear end of the previous sheet is prevented at a position (hereinafter referred to as “pre-registration position”) A where the conveyed sheet is temporarily stopped when the sheet is conveyed to the image forming portion at predetermined timing.
Thus, in the conventional copying machine, the variation in sheet leading end position in the sheet feeding portion is removed by performing the pre-registration operation, and the sheet leading end position is fixed to stably deliver the sheet to the registration roller 108.
In cases where the sheet on the conveying path is temporarily stopped by the pre-registration operation, it is necessary that the next sheet catch up with the previous sheet. Therefore, the conveying speed β in the conveying path from the sheet feeding portion to the registration roller 108 is set faster than the sheet conveying speed (process speed α of registration roller 108) in the image forming portion (see Japanese Patent Application Laid-Open No. 2002-29649).
However, in the conventional image forming apparatus, when productivity of the image formation is improved, the interval between the sheets becomes the narrowest at the pre-registration position. In order to avoid the narrowest interval at the pre-registration position, it is necessary to enhance the sheet conveying speed β as illustrated in a sheet conveyance diagram of FIG. 7.
Therefore, it is necessary to enlarge a driving motor for driving each conveying roller, and it is necessary that a sliding portion for supporting each conveying roller withstand high rotation, which results in cost increase.